Early Development of Olivocerebellar Projections in the Fetal Rat Using CGRP Immunocytochemistry

The expression of calcitonin gene‐related peptide (CGRP) immunoreactivity in certain inferior olivary neurons is transient and developmentally regulated. Labelled neurons begin to appear at embryonic day 16 (E16), and reach their maximal extent by postnatal day 2 (P2). The extinction of the labelling occurs between P13 and P16. Expression of CGRP immunoreactivity is also observed in a few cerebellar fibres from E17, when axons in the restiform bundle begin to enter medially the cerebellar parenchyma. Their maximal extent is reached by P6, and thereafter they slowly disappear following a precise pattern, although fibre extinction is not complete. The spatio‐temporal changes in the olivary distribution of the labelled neurons and the changes in the cerebellar labelled fibres follow the known pattern of topographic arrangement of the olivocerebellar system in adult rats. Moreover, the developmental phases of the CGRP‐labelled fibres in postnatal rats correspond to those known for climbing fibre phenotypic acquisition. Thus, CGRP immunocytochemistry identifies in the fetal rat a subset of inferior olivary neurons and their corresponding cerebellar climbing fibres. Using this approach, we have analysed some of the initial events leading to the formation of the olivocerebellar projection, and obtained the following information: (i) Olivocerebellar axons are not randomly distributed in the restiform bundle before they enter the cerebellum. (ii) In the presence of a large spectrum of choices at the surface of the rostral half of the cerebellar plate the labelled olivary axons begin to enter the cerebellum at a precise medial point to abut a region composed solely of migrating Purkinje cells, and establish contacts with their targets before these neurons reach their final cortical location. (iii) From E18 to E19, the bundle of labelled fibres loses its superficial location, being bypassed by migrating Purkinje cells, to occupy a region corresponding to the prospective white matter. This translocation is coincident with the occurrence of a second axonal entry point, somewhat more lateral than the previous one, and with the appearance of a new lateral stripe of labelled fibres. (iv) Both the early and the late appearing labelled stripes remain confined from the time of their formation in precise cerebellar territories, indicating that only some clusters of Purkinje cells are contacted by the CGRP fibres. The results obtained imply that there is neither a waiting period nor an initial phase of randomness in the formation of the olivocerebellar projection map. This absence of chaotic cerebellar invasion, and the high selectivity of the entry points, suggest that the orientation of CGRP‐positive olivocerebellar fibres towards their targets is regulated by positional information shared between subsets of olivary neurons and clusters of Purkinje cells. The result of this process would be the formation of a precocious coarse topography that would need further refinement.